In a wire electrical discharge machining apparatus, a wire as one electrode is running in an up-down direction and is arranged to be opposed to a workpiece as the other electrode that is controlled to move on a plane perpendicular to the wire running direction. A pulse discharge is caused in a machining gap between the wire and the workpiece (i.e., inter-electrode gap), and the workpiece is machined into a desired shape by utilizing heat energy generated due to the discharge.
In the wire electrical discharge machining apparatus, in a configuration for supplying power to the inter-electrode gap, the workpiece is directly connected to one electrode end of a machining power supply and the running wire is connected to the other electrode of the machining power supply through a feeding point on which the wire is slidable. Generally, two feeding points are provided; one above and the other below the workpiece. In other words, there are two circuits in parallel on upper and lower sides of the workpiece as paths for a discharge current flowing in the wire.
FIG. 6 depicts a configuration of a wire electrical discharge machining apparatus disclosed in Patent Document 1. The conventional wire electrical discharge machining apparatus generally employs, for example, as shown in FIG. 6, two machining power supplies consisting of a sub-discharge power supply for inducing spark discharge (pre-discharge) of small current and a main-discharge power supply for supplying large current as a machining current after generation of the spark discharge to perform rough machining and finish machining.
As shown in FIG. 6, an auxiliary power supply VS, which is a sub-discharge power supply, is connected to an inter-electrode gap (E-W) through a switching transistor Tr1, a coaxial cable W1 as a power supply line, and a resistor R1; and a main power supply VN, which is a main-discharge power supply, is connected to the inter-electrode gap (E-W) through a switching transistor Tr2, a resistor R2, diode D1, a coaxial cable W2 as a power supply line, and an electromagnetic switch K. Generation of discharge in the inter-electrode gap (E-W) is detected in a discharge detecting unit 61, and a pulse controlling unit 62 that receives the detected signal controls on-off operations of the transistors Tr1 and Tr2.
When rough machining is to be performed, the electromagnetic switch K is kept at the closed-circuit state, and the transistor Tr1 is turned on to supply a voltage of the auxiliary power supply VS to the inter-electrode gap (E-W) to generate discharge. After discharge is generated at the inter-electrode gap (E-W), the transistor Tr2 is turned on to supply a voltage of the main power supply VN to the inter-electrode gap (E-W), and rough machining is performed. When fine machining is to be performed, the electromagnetic switch K is turned to the open-circuit state to electrically separate the main power supply VN, and fine machining is performed using only the auxiliary power supply VS.
One of the problems in the wire electrical discharge machining apparatus is how to speed up rough machining. To speed up rough machining, input energy for rough machining has only to be increased, but this leads to wire breakage. The cause of the wire breakage is mainly “concentrated discharge” that discharge is concentrated at one point.
Accordingly, technologies have been conventionally proposed for preventing wire breakage by avoiding the concentrated discharge using a fact that current paths of discharge current exist in parallel at upper and lower sides, as described above (for example, Patent Documents 2 and 3).
Specifically, a technology is disclosed in Patent Document 2 in which by focusing attention on a viewpoint that a difference of currents (current division ratio) supplied from two points, upper and lower, to a discharge position on the wire, which is a resistor, depends on the ratio of the wire lengths to the discharge position, that is, the ratio in accordance with the resistances of wires, current sensors are provided at a feeding point at upper side and a feeding point at lower side, respectively, and the difference of currents output from the two current sensors in accordance with the difference of resistances is detected to measure the discharge position and to stop applying voltage to the inter-electrode gap when the discharge is concentrating.
FIG. 7 shows a configuration of the wire electrical discharge machining apparatus disclosed in Patent Document 3. In Patent Document 3, as shown in FIG. 7, by providing switching elements 71a and 71b capable of controlling independently from each other machining currents supplied from two upper and lower positions, respectively to a wire electrode 70, a technology is disclosed for preventing discharge concentration by making a configuration to supply machining currents asynchronously in the upper and lower sides. With this technology, concentration of currents on one point can be prevented, thereby enabling to prevent wire breakage.
The configuration shown in FIG. 7 is roughly explained. As shown in FIG. 7, the wire electrode 70 runs from upward to downward guided by wire guides 73a and 73b which are arranged in the up-down direction with an appropriate interval therebetween. In a wire running path between these wire guides 73a and 73b, a workpiece 74 is arranged opposed to the wire electrode 70 with a predetermined gap therebetween, and machining liquid nozzles 75a and 75b are provided at positions that sandwich the workpiece 74 from close distances in the up-down direction. These nozzles are provided to remove machining swarf by jetting high-pressure machining liquid to the positions opposed to the workpiece 74 of the wire electrode 70 from the upper and lower sides.
An upper feeding point (conducting terminal) 76a and a lower feeding point (conducting terminal) 76b are provided in sliding contact with the wire electrode 70 at a position near the wire guide 73a and at a position near the wire guide 73b, respectively. Serial-connection-side electrode ends of machining power supplies 77a and 77b arranged in series are directly connected to the workpiece 74. One electrode terminal in the serial circuit of the machining power supplies 77a and 77b is connected to the conducting terminal 76a through a resistor 78a, a switching element 71a, and a diode 79a, and the other electrode terminal in the serial circuit of the machining power supplies 77a and 77b is connected to the conducting terminal 76b through a resistor 78b, a switching element 71b, and a diode 79b. On-off controls of the switching elements 71a and 71b are independently performed by gate-pulse generation circuits 80a and 80b, respectively.
Meanwhile, in wire electrical discharge machining apparatuses, once the discharge is finished, the wire enters in a state with a reaction force received in the opposite direction against the discharge direction. In addition, to remove machining swarf, as described above, jetting high-pressure machining liquids from upper and lower positions that sandwich the workpiece towards the opposed positions is a general practice. However, with the reaction force, machining-liquid jetting, and other effects, the wire vibrates and the straightness accuracy of the workpiece becomes liable to degrade, causing an error in a machining shape. More specifically, another problem of the wire electrical discharge machining apparatus is how to correct the error in the machining shape caused by wire vibration.
To reduce the error in the machining shape caused by wire vibration, one approach is to select parameters such as machining energy, machining speed, wire tension, machining fluid pressure at optimum values for each machining process; however, in the present invention, by focusing attention on the power supply configuration shown in Patent Document 3 (FIG. 7), controlling the machining energy at an optimum value is considered.
Specifically, in the typical power supply configuration shown in FIG. 6, the present invention configures the main power supply VN with two machining power supplies 77a and 77b shown in FIG. 7, and does not connect these power supplies in serial but separates these power supplies into the main power supply for upper-side feeding and the main power supply for lower-side feeding so as to enable to control these power supplies independently. More specifically, the present invention enables to feed the inter-electrode gap independently from each of the upper and lower main power supplies even to make it possible to feed one side from a feeding point of the one side.
In addition, a configuration that also allows a measurement of discharge position as necessary is considered. In this case, because the measurement of discharge position utilizes the current division ratio, as described above (Patent Document 2), when two main-discharge power supplies are used for the measurement of discharge position, the discharge position cannot be measured because the current division ratio cannot be obtained when only one-side main-discharge power supply is used. Therefore, the measurement of discharge position is performed by adopting a configuration to use a sub-discharge power supply. In other words, in order to enable the measurement of discharge position, it is necessary to adopt a configuration to perform feeding from the sub-discharge power supply to the inter-electrode gap using a similar method as the two main-discharge power supplies, as described above, from two upper and lower feeding points. This measure is necessary to prevent useless discharge by stabilizing sub discharge (pre-discharge), which is a spark discharge.
The connections between each one of such two main-discharge power supplies and the inter-electrode gap, and between a sub-discharge power supply and the inter-electrode gap become, for example, as shown in FIG. 8. FIG. 8 is a circuit diagram of an example of configuration for the connections between each one of the two main-discharge power supplies and the inter-electrode gap, and between the sub-discharge power supply and the inter-electrode gap using a conventional technology when configuring a wire discharge machining apparatus to enable the measurement of the discharge position by using the two independent main discharge power supplies for an upper-side feeding and an lower-side feeding, and by using the sub-discharge power supply.
As shown in FIG. 8, the wire discharge machining apparatus aimed by the present invention includes a main-discharge power supply 85a for upper-side feeding and a main-discharge power supply 85b for lower-side feeding that can be controlled independently with each other and a sub-discharge power supply 86 as a machining power supply for the discharge machining unit shown in FIG. 7 (the wire electrode 70, the workpiece 74, the upper and lower-side feeding points (conducting terminals) 76a and 76b, and the upper and lower wire guides 73a and 73b; meanwhile, the machining liquid nozzles 75a and 75b are abbreviated in the figure). An upper terminal block 87a and a lower terminal block 87b are provided to connect between each one of the two main-discharge power supplies 85a and 85b and the inter-electrode gap, and between the sub-discharge power supply 86 and the inter-electrode gap as described below.
More specifically, at the upper terminal block 87a, its wire electrode connection end E is connected to the upper feeding point (conducting terminal) 76a, on the other hand, one electrode end of the main-discharge power supply 85a and one electrode end of the sub-discharge power supply 86 are each connected to this wire electrode connection end E. On the other hand, at the lower terminal block 87b, its wire electrode connection end E is connected to the lower feeding point (conducting terminal) 76b, on the other hand, one electrode end of the main-discharge power supply 85b and one electrode end of the sub-discharge power supply 86 are each connected to this wire electrode connection end E. A workpiece connection end W of the upper terminal block 87a and a workpiece connection end W of the lower terminal block 87b are each connected to the workpiece 74, on the other hand, each the other electrode end of the main-discharge power supplies 85a and 85b and the other electrode end of the sub-discharge power supply 86 are each connected to each workpiece connection end W of both terminal blocks.
If each one of the two main-discharge power supplies 85a and 85b and the inter-electrode gap, and the sub-discharge power supply 86 and the inter-electrode gap are connected as described above, during one-side feeding using one main-discharge power supply, feeding from the sub-discharge power supply 86 to the inter-electrode gap can be performed from two upper and lower points of the wire electrode 70, so although a current sensor is not shown in FIG. 8, if current sensors are provided at the two upper and lower points of the wire electrode 70, the measurement of discharge position using sub discharge current becomes possible.    Patent Document 1: Japanese Patent Application Laid-open No. H7-276142 (FIG. 5)    Patent Document 1: Japanese Patent Application Laid-open No. S61-15017 (FIG. 1)    Patent Document 3: Japanese Patent Application Laid-open No. H1-97525 (FIG. 2)